DEVELOPMENT 197 



double- walled, each wall being many cells in thickness. (Here 

 we neglect the peculiar " choroidal " fissure.) At this phase the 

 cells in the integument, just over the opening of the optic cup, 

 thicken (8), form a little rounded body (7) which then becomes 

 detached from the integumentary epithelium and comes to lie 

 in the opening of the optic cup (at 6). This small, rounded body 

 becomes the crystalline lens of the eye. Plainly we have now 

 an optic anlage displaying all the structural plan of the vertebrate 

 eye. In it all the cells are embryonic, or undifferentiated ones. 

 It has come into existence by means of cell-divisions of an original, 

 formative epithelium, which divisions have tendencies. 



All organs are " blocked-out " in such ways — by cell- divisions 

 that occur so that as the cells are formed they are marshalled 

 into place in definite tectonic arrangements. The materials of 

 these cells come from the nutritive substances at the disposal 

 of the embryo. For simplicity we speak of cells as if they were 

 separate bodies, but actually they remain in contact, or even in 

 structural continuity with each other and their boundaries, or 

 " walls," may even be obscure. They may be regarded as 

 structural elements, or " building-stones " made on the spot as 

 required. The description that we give of the formation of an 

 organ-anlage does not, at all, " explain " that formative process : 

 what has to be explained is the agency that divides the cells at 

 the appropriate places and in the appropriate planes and that is 

 the problem of development. But even the description is com- 

 plex, though on practical acquaintance with an embryogenic 

 process, it is very easy to follow. 



71. ON EMBRYOGENY : II. HISTOGENESIS 

 When organogenesis has proceeded so far as the formation of 

 the anlagen the cells are still embryonic, or undifferentiated in 

 type. The eye, as a functioning organ, contains skeletal, connect- 

 ive, muscular, nervous, receptor, glandular cells, etc., but its 

 anlage, as we have seen it develop, consists only of small rounded 

 cells that are all similar to each other. As development proceeds 

 these cells differentiate into categories and each category consists 

 of cells that are tissue-elements. Thus there may be cartilage- 

 cells, muscle-cells, the nervous cells that form the retina ; the 

 nerve-fibres ; cells that make up blood-vessels ; the transparent 



